The present invention relates to apparatus for controlling gas flows and more particularly to apparatus for permitting a gas flow to continue for only a predetermined time period.
There are many instances in which it is necessary to initiate a gas flow for a limited period of time in response to the sensing of a particular condition. For example, in the metal heat treating industry wherein parts are subjected to heat treating operations in a furnace, it is known to supply a flow of purge gas, typically an inert gas, to the furnace upon detection of abnormal conditions therein. For example, in the event the furnace temperature drops below a predetermined level or the oxygen content of the furnace atmosphere exceeds a predetermined value, it is common for safety reasons to simply flood the furnace with a flow of inert or purge gas thereby assuring that safe conditions are retained in the furnace.
Frequently, it is desired to provide for an automatic initiation of a purge gas flow in response to detection of a condition as mentioned above and then to have an operator terminate the flow of purge gas manually when the need for such flow no longer exists. However, in many industrial applications, particular furnaces or other chambers or vessels to which a purge gas flow may be applied are left unattended for significant periods of time. Typically, a furnace may be provided with a certain degree of heat over a weekend or the like to reduce the time necessary to recommence operation. In the event that furnace temperature is lost, or oxygen levels increase to an undesired extent while the furnace is unattended and detection equipment is effective to cause the flow of purge gas to the furnace and to shut the same down, it is economically highly desirable to terminate the flow of the purge gas to avoid wasting the same by unnecessarily introducing the purge gas into the furnace after the same is required for safety purposes.
It is known to those skilled in the art to utilize certain cycle timing devices for controlling gas flows and typically, a mechanical cycle timer may be utilized to supply a gas flow for a predetermined time interval, terminate such flow for a second interval and then reestablish the flow for the first time interval and to repeat the same sequence of steps. These cycle timing systems are relatively complex and rely on numerous moving parts in order to open and close a valve, on an intermittent basis, for predetermined time intervals as, for example, is shown in U.S. Pat. No. 3,211,179. It is, however, known to use pneumatic cycle timing systems as described in U.S. Pat. Nos. 2,760,511 and 3,326,237. These devices are effective to generally control the duty cycle or duration of pulses of a gas flow and typically, such devices utilize gas storage reservoirs in addition to a separate gas supply to provide gas flows at a given rate for a predetermined time interval and to interrupt such flows for another predetermined interval. However, as these cycling devices continually provide intermittent gas flows of particular flow rates, the same are not suitable for use in simply terminating a gas flow after a predetermined time interval has elapsed.
Other attempts to control gas flows, such as purging gases, have involved the use of complex electronic systems such as the apparatus illustrated in U.S. Pat. Nos. 4,064,898 and 4,270,564. In these systems, the duration of a gas flow is controlled by generating electrical pulses at a particular rate and counting the same such that upon reaching a desired count, a valve in a gas flow conduit will be actuated. This apparatus is, however, complex and expensive and does not readily lend itself to widespread use in relatively heavy industrial applications such as in connection with control of flows of a purge gas supplied to heat treating furnaces or the like.
Thus, the prior art exhibits a clear need for relatively simple, inexpensive and reliable apparatus for terminating a gas flow after the same has continued for a predetermined time interval.